1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition and a ceramic piezoelectric device. More particularly, the invention relates to a piezoelectric ceramic composition serving as a useful material for producing ceramic piezoelectric devices such as piezoelectric filters, piezoelectric oscillators, and piezoelectric transducers, and a ceramic piezoelectric device employing the ceramic composition.
2. Background Art
Conventionally, piezoelectric ceramic compositions containing lead titanate zirconate (Pb(TixZr1xe2x88x92x)O3) or lead titanate (PbTiO3) as a primary component have been widely used for producing piezoelectric devices such as piezoelectric filters, piezoelectric oscillators, and piezoelectric transducers. However, since such piezoelectric ceramic compositions contain a large amount of lead, lead oxide is evaporated during the production processes of the mentioned products, posing a problem that product quality varies from product to product. In order to solve this problem, the dielectric ceramic compositions preferably contain a small amount of lead, and more preferably no lead.
In contrast, piezoelectric ceramic compositions predominantly containing a bismuth layered compound such as CaBi4Ti4O15 suffer no such drawbacks due to the absence of lead oxide in the composition.
However, piezoelectric ceramic compositions predominantly containing a bismuth layered compound such as CaBi4Ti4O15 must be fired at a temperature as high as 1150xc2x0 C. or more so as to obtain piezoelectric ceramic material exhibiting an electromechanical coupling coefficient (kt) of 10% or higher, which is needed for practical use. Firing at such high temperatures requires a high-performance firing furnace of high cost. In addition, when a ceramic piezoelectric device such as a piezoelectric filter having internal electrodes is produced, the electrode material must have a melting point temperature higher than the firing temperature of the material per se. Thus, expensive electrode material such as platinum or palladium must be used, thereby increasing production costs.
In view of the foregoing, an object of the present invention is to provide a piezoelectric ceramic composition predominantly comprising CaBi4Ti4O15 which composition can be fired at 1100xc2x0 C. or lower, which exhibits an electromechanical coupling coefficient (kt) sufficient for practical use, and which serves as a useful material for producing ceramic piezoelectric devices such as piezoelectric filters, piezoelectric oscillators and piezoelectric transducers. Another object of the present invention is to provide a ceramic piezoelectric device employing the ceramic composition.
Accordingly, the present invention provides a piezoelectric ceramic composition comprising a primary component represented by the formula CaBi4Ti4O15, wherein the composition contains W in an amount of about 0.5 mol or less (but greater than 0) per mol of Bi contained in the primary component. Preferably, the composition contains a primary component represented by CaBi4(Ti1-yWy)4O15 (where 0 less than yxe2x89xa60.5).
Preferably, the piezoelectric ceramic composition contains a divalent metal element other than Ca in an amount of about 0.125 mol or less (but greater than 0 mol) per mol of Bi. The divalent metal element may be at least one Mg, Sr and Ba.
Preferably, the piezoelectric ceramic composition contains a trivalent metal element other than Bi in an amount of about 0.075 mol or less (but greater than 0 mol) per mol of Bi. The trivalent metal element may be at least one of La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, Sc and Y.
Furthermore, the piezoelectric ceramic composition preferably contains Mn in an amount of about 1.5 wt. % or less (but greater than 0) calculated as MnCO3.
In the present invention, there is also provided a piezoelectric ceramic material comprising a piezoelectric ceramic composition as recited above and a ceramic piezoelectric device containing electrodes attached to the ceramic material.